My guess is that their market domination will come to an end 'soon'. Nobody wanted to switch to Google+ or anything else because they would have to persuade all their friends to move at the same time (and therefore all their friends' friends). Just not going to happen.

The break will come when someone creates a social networking site that also aggregates the feeds from other sites (like Facebook) and relays postings in an agreeable manner. So for example I log into Google++, see my friend's Facebook status update, comment on it and the system logs into FB on my behalf and passes on the comment. It's almost trivial in terms of programming. As soon as a site like that gets it right and catches on, Facebook will be demoted.

I bet this is one of those fleeting thoughts that I see realised a few years later and which makes the 'inventor' rich, while I sit at the bar saying "I thought of that!" to anyone who will listen. Like Tivo.